Regen braking: confirmed!
 

I was doing a little playing around with my EZRun80 ESC the other night. I still need a motor mount, but I thought I'd get some data from an unloaded motor.

Before I go on, let me make the disclaimer that I do NOT suggest ever running a motor unloaded! Do so at your own risk!

I hooked a freshly charged 4s lipo pack and a Neu 1512/2d to the ESC. I also attached a Fluke 87A voltmeter to the battery wires (using a little adaptor I made). I then set the meter to "record mode" where it will take samples. In this mode, the meter will also record the min, max, and average readings. Nort sure what the sampling rate it though.

Remember, the motor is not attached to anything.

I gradually applied throttle until rpms were at max. Lipo voltage at this point was about 14.9v. Then, I jammed on the brake. When I looked at the meter's max voltage, it read 29.4v! Yeah, there's regenerative braking here, and quite a bit of it. Repeating the test gave me the same approximate result.

Then I asked myself "self, I wonder what the current is at that point?". I had to be careful here since my meter is only rated for 10A, so I changed the leads around so the meter was in series between the ESC and battery, and reset the recording mode. Again, I gradually increased throttle to almost max (to reduce current spikes), and then jammed on the brake. I measured -2.96A. Repeating the test got -1.82A. Yes, negative current. That's because my meter was set up to read current to the ESC, not from it.

Obviously, the spike is very brief and I am probably not getting the full reading due to my meter's sampling rate, but those are pretty hefty numbers from an unloaded motor, which means there is a LOT less momentum acting on the windings/magnet than there would be in a moving vehicle. I shudder to think what those poor caps and FETs deal with in a vehicle with a LOT more kinetic energy!

I wonder if all ESCs should have a transorb or something that would shunt any voltage no more than 10% higher than the cap ratings? These spikes, even as brief as they are, can't be good for components! IIRC, the MMM does have these devices which kick in at 26v, so that makes me feel a little better.

Anyway, just thought some would like to see real test results. If anything, this proves regenerative braking IS present, at least in the HobbyWing EZRun80, but the design is just like just about every other ESC on the market, so I think we can assume all the other ESCs act the same way.

And yes, motor and ESC are both still fine.

Good info here. So the EZRun has regenerative braking. Does the Castle Creations Sidewinder have that?

Yes they have to put the power back to the batteries as the ESC can't take it all

Oh, so theoretically, its "charging" your battery pack?

Yup! Neat, eh? :smile:

sweet, just like the 1:1's do!!

does the mm do this?

You will also notice regenerative braking when hooking up an eagletree logger to a car setup.

http://www.braintrust.at/2007/ds/rc/...remsdetail.gif

Driving my truggy on asphalt with a Neu 1512/2Y (1400kv), Jazz 55 ESC and 10s1p A123. I marked the amp and voltage peaks where i applied brakes. The green line represents motor rpms. The eagletree logger records positive Amps (not negative amps) and voltage does rise instead of falling -> the battery is recharged. Amp Peaks around 15-20A with voltage rising by 1.3-1.4V

A similar behaviour can be seen with MMM and BK ESC.

Quote from Pdelcast about MMM Esc

Lipos are supposed to be charged with constant current/constant voltage correct? If so do these short burst of varying voltage/current have any negative effects on the lipos?

The duration of those spikes are pretty short, so it should be ok.

Now that's cool! I always thought the voltage spike going up as the amp draw comes down was just the battery responding to the reduced load placed on it, but it makes sense also that there should be some current flowing back into the batt.

Great work champ ;)

Just for the record, I would anticipate a smaller voltage spike, but larger current flow when braking hard in a vehicle.

Because an unloaded motor stops almost instantly (only the inertia of the rotor/shaft to stop), the magnetic field collapses instantly as well, which induces a large voltage. But since a field only has so much energy (think "watts"), a high voltage will yield small current.

In a vehicle, the motor will not stop as instantly (vehicle weight "drags" the motor further), so the field will collapse a little more slowly yielding a little less induced voltage, but the spike width would be a little longer and also flow a little more current. This is confirmed by othello's graphs above.

Isn't it nice when all the theory and formulas agree with real-world results? :smile:

This test was just for fun really, but it does prove that back-EMF can in fact produce a higher voltage than the supply (battery). It's all about the time the field expands vs the time it is allowed to collapse.

Now, this back-EMF spike will be dependant on several factors: vehicle weight, speed just before braking, motor kv, motor style (slotted vs slotless), charge voltage (battery), traction of surface, etc.

This is cool stuff, does anybody know if the MM does this?

Yes, ESCs are simply not big enough to absorb that amount of energy in such a small package, so it has to go somewhere...

This is cool Brian. I always wanted to do this test myself but never really had a good enough multimeter to do it. Especially after a little experiment I did myself a couple weeks ago. I have actually been meaning to start a thread on this very topic. I have heard for a long time that they did have regenerative braking in just about all ESCs but never saw any proof of the fact.



I'm hopeing someone can answer why it is that I see 10-15% better milage when I run mechanical brakes? This is on the same truck on the same day on the same track with the same setup and batteries. I did some testing the last time I went out for a practice day and wanted to see what effect the mechanical vs motor brakes had on run time. I ran 2 5 minute runs with the mechanical brakes on 2 different packs, and then took the mechanical brake off, and charged the packs back up recording the amount of energy I put back in each one. Then ran two 5 minute runs with the mechanical brakes taken off and used motor brakes. I tried to drive just as hard with both brake setups and ran 2 trials on each just to add a little extra data.

But for sure, both runs with the motor brakes used more mah from the packs than the runs with the mechanical brakes did.

A day or 2 after I actually did this I realized I could have used my eagle tree to get better data. Next time I will do it that way.



I just dont get it, the evidence would support the opposite effect being true. Longer run time with motor brakes.:neutral:

Wondering if anyone has any thoughts?

The motor is still using power to stop. In other words, ESC braking is not free. The 'regenerative' part is that it can recapture small (if not tiny) amounts of power in between the pulses while the braking power is applied. After all, a force is essentially being applied in the opposite direction to bring the car to a stop, just as it would if it was starting from a standstill. This is also why motor brake setups run hotter since the motor is working in both directions. Also, braking force is usually applied more abruptly than while accelerating. AFAIK, a single mechanism (the motor) can't function as a generator and a source of power at the same time. You must use power to brake the car. I'm not an electrical engineer, but I know a few things about physics and the forces that are present and being applied in this scenario simply can't be ignored.

Please correct me if I'm wrong here because our results coincide with jhautz and others that have found mechanical brakes to be more efficient. I'm not arguing that some energy isn't being harnessed or regenerated while braking, I'm saying that it does in fact use power to brake and that the overall net of the system is negative, not positive. Mechanical brakes are obviously using power as well (servo), but compare a servo's energy requirements to a 1/8th brushless motor's requirements and I think the picture becomes clearer.

Thats what I was always thinking. I can see how coasting off power would actually spin the motor and make it act like a generator. That makes sense to me. But braking to me always seemed like it would take energy to offset the energy that the truck already had in its momentum.

Well I'm not disagreeing 100% but have you ever tried shorting a brushed motor - No power input is actually applied but almost impossible to turn over - This is the easiest way to dig a clod....

You don't need to have power to provide breaks when using a motor...

Patrick is the one we need to chime in here...

PS http://www.ezonemag.com/pages/faq/a504.shtml

That would be nice. Someone who actually knows how these thigs work. lol

Actualy Its consume very little not more than couple amps! One guy in russian forum made tests about regenerative brakeng and he did not recorded any noticable power consumption its just shorting motor windings and power force applies through shorting Impulse lenght and opposite phase switching, power consume mostly for FET drivers, brains then braking. MGM, MMM, MM, Schulze, Quark B125. No one regenerated a power, all regenerated power goes for FET heating.

Please don't misunderstand. I'm not saying that regenerating isn't happening while braking. There is stored energy in the vehicle to be recaptured. But unless the regeneration or conversion of the stored kinetic energy is 100% efficient, some other energy must be used to stop the car, especially if you are braking hard at high speed. If the conversion was 100% efficient we'd have a perpetual motion machine in our RC cars. Shorted motor resistance is one thing, but we all know how motor brakes can flip a car on it's lid. If shorting was that powerful you wouldn't be able to turn a brushless motor shaft by hand if all three wires were touching each other. This is why I think there is power being applied while braking.

Again, I'm not an electrical engineer, maybe at higher speeds, the shorting becomes more powerful and violent enough to flip the car, not sure. This still doesn't explain why a lot of people are experiencing better run times using mechanical brakes though. I'd like to know for sure from someone who can explain it as well. I've read up on full scale regenerative braking and they all have mechanical brakes to supplement the regenerative brakes for various reasons. One of the reasons cited was to more quickly bring the vehicle to a stop from higher speeds.

So, please, someone explain. If motor braking is indeed free, I have a couple things I want to try to get even more run time :smile:

Confusing.
 

AFAIK, a single mechanism (the motor) can't function as a generator and a source of power at the same time. You must use power to brake the car. I'm not an electrical engineer, but I know a few things about physics and the forces that are present and being applied in this scenario simply can't be ignored.

During my 20 years in the Navy I have seen on a couple of occasions where I generator which normally puts out huge amounts of power was turned into a motor when the electrical power flow was reversed accidently. Due to the circuitry involved within the system it was not supposed to happen, but it did.

There is no way that electrical braking is accomplished without using some source of power. That's just a relitive impossibility. But as you stated Patric can probably explain this so we can all understand it.

Why you cant brake without power, ofcourse you can by just shorting windings on motor or put some capacitance (capacitors) to windings thats it, maybe its not so effective. Maybe you cant change brake force without power source, somehow you just need change chorting impulse lengt and opposite phase switching, that basicaly any ESC do.

I will speak with guy who made a tests on ESC to check Ezrun 150A, does energy recuperation braking works there?

When i first tryed my eagletree, I used it on a MM/9xl powered G2R. I recorded a spike of over 269 amps. It to was instantaneous, and saw a voltage spike, but not a big one, maybe 3 volts or so in the same space. I thought it was a glitch or a bad reading, but this made me think about it again. That's some extreme amps, but possibly the same effect. Makes me wish i had saved that file to show, I thought it was a bad reading

The mighty A-gangers did it again :rofl::rofl:.

We lost power to the whole ship during my last deployment due to this. We basically shorted the diesel generator.

Anyway, whatever happens, energy is not lost or destroyed (First Law). However, it is transformed into another form of energy which is basically heat. Now you wonder why motor braking produces a hotter motor compared to mech brakes :whistle::whistle:.

A looong time ago, I too thought that motor braking was achieved by the ESC partially shorting the phases via PWM. Due to the way ESC FETs are wired to the motors, this would be done by turning on all the FETs going to ground. But, doing this would generate no voltage/current. And as you can see via my original post, the ESC does generate both voltage and current. The numbers don't lie boys. The fact that I got a higher voltage than the supply (battery) and negative current (current flowing into the battery rather than from the battery) both indicate, without a doubt, that there IS regenerative braking.

However, Patrick has said the ESC sends the motor's kinetic back into the ESC and then to the battery. If this is happening (and it is), simply shorting the windings would not accomplish this. Shorting the windings would brake the vehicle, but where is all that kinetic energy going? I'll tell you; either the motor and/or ESC. Neither are sized to simply dissipate that power.

There is a graph (in a previous post) by othello which proves regen braking as well.

Even Mike agrees with the results. He has Eagletree graphs where the mAh consumed is greater than the battery capacity. As well all know, when you have any decent current flow, the mAh used will actually be LESS than battery capacity. So, even though ET doesn't show negative current (does show v spikes though), just the fact you get more mAh is a giant red flag in favor of regen braking.

jhautz: why you get less runtime with motor braking is still puzzling to me. Without a consistent and repeatable test method, results are simply objective. Not trying to shoot ya down, just my $0.02.

I also think people are getting confused between energy transfer in this thread.

The car when moving has kinetic energy : E = Mass * Velocity * Velocity

That kinetic energy can only either be transferred into heat, electrical energy or potential energy (coasting up a hill) (ok light too)...

In the case of a mechanical brake it is always transferred into heat in the brake pad whilst with motor breaks some is transferred into electrical energy.

Second consideration is Newton - i.e. equal and opposite forces required to stop the vehicle.

In a mechanical setup the force is supplied by brake disk which is pressured by a servo which is powered by the battery.

In an electrical setup the force is supplied by the motor - This is where is gets interesting! How is the force generated!?*&^!

Option 1) Through converting the motor into a generator? Which would generate electricity and through losses heat but "not take" energy from the battery
Option 2) Applying power to the motor with an opposing polarity which would "take" energy from the battery
Option 3) Or a fancy combination of both...

Guessing how smart patrick is it is 3... Thats my take...

Could it be the result of the quality of the battery packs in use? If low quality packs with higher internal resistance can't take back the load they're feeded during braking, the energy is transformed into heat instead of actually recharging the pack.

Thats is one thing that wouldn't be an issue with one of Jeff's setups - He's a uber quality build guy!

Option 3 is my guess too. I acknowledge that regenerative braking is happening (I'm really not arguing that point, reread my post:smile:). Brian's test proves there is extra energy for a brief time while braking. But I think some other power or force is also needed to stop the car in most situations, that's all I am trying to say.

I think that regen can be more of a marketing thing than anything else. What I've noticed is that the ESC will send send energy to the motor to brake it and that can be seen in most graphs, but there might not be any AMPs going back to the battery. Our little motors as you guys know are basically like an alternator and can be used to make electricity. An alternator can be modified to be a motor and I've seen some. Even in a full size electric vehicle, regen is more for marketing. Now what is different about a full size car is that they can coast much more then our R/C vehicles and that might help them put back a little energy into their pack, but not enough to give it a good charge. So regen braking is more marketing then something practical for our hobby. Brian, a good test to see is spinning a brushless motor and see what kind of reading you get from it.

For sure, the regen braking doesn't do a whole lot to add to runtime. I think the main reason is to provide a place for the energy to go, the added runtime just happens to be a collateral benefit. :smile:

Way ahead of you lutach: http://www.rc-monster.com/forum/showthread.php?t=4997

It appears I didn't post it, but I do remember making a three-phase bridge rectifier and capacitor filter and it did provide a decent amount of power (don't have the numbers though).

iirc patrick mentioned up to 35% eff - note this is of the braking effort only...

I don't really think of it as a marketing thing as actually is saves the ESC / Motor from heating up...

Yes, I did see that one, but it kind of slipped my mind with so many other good threads. I don't thing for the millisecond of this regen it would be beneficial for adding any amount of energy back into the pack. The moment we use the brakes to slow down we basically loose more energy mainly because it will take more to get up to speed.

What I meant by marketing is when a company uses it as a form to sale the product. Yes, the energy does go to the place it came from and it might loose a little due to wires, connectors, resistors, traces, capacitors and more wires and connectors. So pack might not see anything at all in a way if you count the losses for the regen energy to get there.

Agree - I've never seen castle or anyone touting "regen breaking"... But do think that it is a key item and differentiator in enabling the ESC and motor not to heat up...

I agree with tekno that it is obviously going to be cooler without motor breaking as the energy will be lost as heat in the disc/pads rather than the motor.

Of course, but you have to get back up to speed whether you use motor or mechanical brakes. It's obvious that the amount you put back in during braking will be FAR less than you use to accelerate, but a few more mAh is a few more mAh, and maybe a couple seconds more runtime. :smile:

Believe me, the intent of the original post was not to start a heated debate about gains in runtime using motor braking. It was just me playing around and tinkering. But, I have seen several posts in the past where this question was asked, so I thought it would be nice to have a tested result instead of theorized.

Brian - this is one of the more fun threads for me - a chance to use a fraction of my engineering background!

I like things like this. The more the better and a heated debate will never come up in here. I've also seen post asking this question and this should keep people informed. This is a nice thread, you got an A+ :lol:.